7. THE MANTLE ROCKS 



J. I. EWING 



1. Introduction 



The Earth's mantle is larger in volume by a factor of 10 than the core and 

 crust combined. Its outer surface is found at an average depth of 12 km under 

 the ocean basins and at about 35-50 km under the continents. The depth is 

 intermediate in intermediate areas. The inner surface (the core-mantle dis- 

 continuity) is about halfway between the surface and the center of the earth. 

 The composition of the mantle is thought to be principally olivine, on the basis 

 of seismic wave velocity, density and examination of materials brought up in 

 oceanic volcanoes. For a review of the variation of seismic velocity with depth, 

 based on the study of earthquake waves, the reader is referred to Chapter 8, 



The present discussion is concerned with the information about mantle 

 structure under the oceans derived from seismic refraction studies (see Chapters 

 1 and 2). To date, the length of marine seismic profiles has not exceeded 200 km 

 and has generally been less than 100 km ; hence the velocity measurements 

 have not extended to any appreciable depth in the mantle as in earthquake 

 seismology, but rather have sampled only the upper part. This offers at least 

 one advantage in studying the nature of the crust-mantle (M) discontinuity. 

 Even with the detail obtained by surface-wave dispersion analyses, the results 

 from earthquake seismology show the average structure over a path that is 

 usually thousands of kilometers long. Seismic refraction measurements give 

 results in considerable detail from a local area, hence comparisons can be made 

 from one place to another and certain conclusions about the homogeneity can 

 be drawn from the observed differences or similarities. 



2. Compressional-Wave Measurements 



In Fig. 1 are plotted approximately 180 recorded seismic compressional- 

 wave velocities higher than 7 km/sec from profiles in the Atlantic, Pacific and 

 Indian Oceans, the Caribbean Sea and the Gulf of Mexico. Only those velocities 

 were plotted which were measiu-ed at profiles where an overlying layer of high- 

 velocity crustal rock (6-7 km/sec velocity) was present. This eliminated Mid- 

 Atlantic Ridge profiles and some others on continental margins. These values 

 come from Officer et al. (1952), Hersey et al. (1952), Ewing et al. (1954), Hill 

 and Laughton (1954), Ewing et al. (1955), Katz and Ewing (1956), Raitt 

 (1956), Gaskell et al. (1958), Hersey et al. (1959), Ewing and Ewing (1959) 

 and unpublished results by Ewing et al. and Hennion and Ewing (MS). A 

 quick look at Fig. 1 indicates the problem of defining the mantle rocks by 

 seismic velocity. There is no doubt that a peak in the frequency plot occurs at 

 about 8.1-8.2 km/sec, and this is in good agreement with the generally accepted 

 value of Pn from earthquake seismology. As Hill (1957) pointed out in a 

 review of recent explorations of the ocean floor, this would at first sight seem 

 to indicate that the rocks with compressional-wave velocity 8.1 km/sec and 



\_MS received August, 1960] 103 



